1. Field of the Invention
The invention relates to integrated circuits incorporating amplifiers.
With the development of MOS and CMOS technologies, the present trend is towards the integration of increasingly complex systems that combine both logic and analog functions on one and the same integrated circuit chip.
The problems that arise then relate to interference between the logic part and the analog part, i.e. the noise-infestation of the analog functions by the logic signals that travel through neighbouring conductors.
The primary cause of this noise is the switching over of logic signals between two voltage levels that are relatively distant from one another. The transients thus generated are passed on capacitively or inductively to neighbouring conductors which they disturb. The noise thus generated covers a wide spectrum of frequencies since the logic level transitions may be very steep and the logic level steps may be flat.
The disturbed conductors include, for example, the supply (Vcc and Vss) lines. But there may be other circuit elements such as, for example, the substrate of the integrated circuit itself: when the potential of the substrate is transiently altered by noise, this influence is reflected in the active devices which have channels continuous with the substrate.
In particular, in analog functions (amplifiers for example), the logic noise generated by the neighbouring logic functions may give rise to an instability of the reference voltages internal to the circuit, whether it is the reference of the substrate or a supply voltage; or quite simply the point of operation at rest of a circuit node.
2. Description of the Prior Art
To compensate for these instabilities, it is often necessary to provide compensation means. For example, the most frequently used method consists in setting up (noise-free) reference current sources externally to the integrated circuit, to provide the noise-free references needed. In some cases, integrated circuit configurations are also used, with separate supplies provided for the analog part and for the logic part of the circuit, so that interference is minimized.
One of the problems encountered then is that there is an increase in the number of external pins of the integrated circuit. It is known that it is desirable to reduce the number of external pins of an integrated circuit to the greatest possible extent.